1. Field of the Invention
The present invention relates to an image reading apparatus and an image reading method for reading images on both sides of a document during a single conveyance of the document, and a recording medium recording a program for executing the image reading method.
2. Description of the Related Art
Examples of image reading apparatuses widely used and capable of electronically reading image information of a document include a copier, a facsimile, a multifunction copier having a facsimile function, and a scanner for reading images to be input to a computer. There are two types of image reading apparatuses: one is an apparatus in which a fixed document is read by moving a light source, while the other is an apparatus in which a moving document is read by a fixed light source. Examples of the latter include an apparatus in which a plurality of documents are automatically conveyed and continuously read, and an apparatus in which a double-sided document is internally reversed so that both sides of the document are automatically read without any manual intervention.
Here, a double-sided document means a document with image information (e.g., text, graphics) printed on both the front and back sides of the document.
Additionally, with recent development of compact high-performance devices, there has become available a double-side simultaneous image reading apparatus capable of simultaneously reading both sides of a document without reversing it. The double-side simultaneous image reading apparatus includes two reading devices for respective front and back sides of a document and is configured, for example, as illustrated in FIG. 1.
Referring to FIG. 1, the image reading apparatus includes a document plate 101, a pickup roller 102, a conveying roller 103, driven rollers 104, light sources 105 and 107, reading parts 106 and 108, and a glass table 109 serving as a document table. In the image reading apparatus, a first reading device 110 includes the light source 105 and the reading part 106, while a second reading device 111 includes the light source 107 and the reading part 108.
Double-sided documents loaded on the document plate 101 with their front sides facing up are fed one by one by the pickup roller 102 to a reading path. A picked-up document is conveyed by the conveying roller 103 through the driven rollers 104 to the interior of the apparatus. The reading part 106 is provided with the light source 105. The light source 105 has a spectral intensity in a wavelength range of visible light.
A document side (front side) of the document having reached a reading position of the first reading device 110 is irradiated by the light source 105. The light reflected off the document by the irradiation is incident on the reading part 106. Similarly, the opposite (or other) document side (back side) of the document having reached a reading position of the second reading device 111 is irradiated by the light source 107. The light reflected off the document by the irradiation is incident on the reading part 108.
The reading parts 106 and 108 each have at least a photoelectric conversion element that accumulates electric charge of an amount corresponding to the intensity of incident light. The accumulated electric charge is converted by an analog-to-digital (A/D) converter (not shown) from an analog signal to a digital signal. Thus, the image information on the document is converted to digital image data. The intensity of light incident on the reading parts 106 and 108 depends on the distribution of spectral reflectivity included in the information on the document.
As described above, the front side of the double-sided document conveyed to the reading position of the first reading device 110 is read by the light source 105 and the reading part 106, while the back side of the double-sided document conveyed to the reading position of the second reading device 111 is read by the light source 107 and the reading part 108. Here, the front side refers to a document side in a single-side reading mode, while the back side refers to a back side in a single-side reading mode.
Advantages of simultaneously reading image information on both sides of a double-sided document in the double-side simultaneous image reading apparatus described above are that no user intervention is necessary, the reading operation is quicker than in the case where the document is reversed, and the occurrence of jamming is reduced because only one document conveying path is provided.
At the same time, however, there are some disadvantages as follows. First, since reading devices for the front and back sides of the document are different, there are individual differences between the reading devices. The degree of degradation in durability after use for a certain period of time will also be different between the reading devices. Additionally, because of a difference in distance by which the document floats from the conveying path at the reading position, the modulation transfer functions (MTFs) of the reading devices are different. In other words, color and geometric characteristics of images read from both sides of the document may be different. Moreover, if the reading devices for the front and back sides are those having different configurations, such as a reduction optical system (charge-coupled device (CCD) sensor system) and an equal-magnification optical system (contact image sensor (CIS) system), differences in the above-described characteristics may become more evident.
If one image reading apparatus provides different reading characteristics for the front and back sides of a document, the user will inevitably mistrust the apparatus. Therefore, in an image reading apparatus capable of reading both sides of a document, it is at least necessary that reading characteristics for the front and back sides of the document match.
For example, Japanese Patent Laid-Open No. 2003-032504 proposes a calibration method to address the problems described above. In this method, the same reference pattern is read by a first reading unit and a second reading unit. Then, to reduce a difference between the output values of the first and second reading units, RGB-γ conversion tables for the first and second reading units are corrected.
In the method proposed in Japanese Patent Laid-Open No. 2003-032504, it is possible to match the reading characteristics of two different scanners. However, since a change in color before and after calibration is not taken into account, the reading characteristic may be changed significantly depending on the state of the scanner.
Japanese Patent Laid-Open No. 2005-244519 presents a proposal based on considerations for a change in color before and after calibration. That is, a calibration which involves control of the amount of correction is proposed. The proposal of Japanese Patent Laid-Open No. 2005-244519 discusses calibration in a single-side scanner and places its primary focus on reduction in operation time. A calibration apparatus proposed in Japanese Patent Laid-Open No. 2005-244519 includes a control unit which performs control to prevent a sudden change in color before and after correction. A correction chart is read by the scanner, colorimetric measurement is performed, and calibration is performed by using the colorimetric value as a target. If the amount of correction exceeds a predetermined value, the amount of correction is not added to the colorimetric value serving as a target. Then, correction is made only by an amount within a predetermined range. By performing control to reduce intervals between calibrations and repeating the above-described calibration many times, a calibration using the colorimetric value as a target can be eventually made. However, an ideal state cannot be approached without repeating the calibration itself many times. Therefore, if a device does not have an automatic colorimetric measurement unit, it is necessary to perform colorimetric measurement every time and thus to spend much effort.
The calibration presented in Japanese Patent Laid-Open No. 2005-244519 is based on colorimetric measurement. However, even when a colorimetric value is used as a target for calibration, there are still some problems to be solved. Since a reference level serving as a final target is gradually approached, it takes a long time to match the reading characteristics for the front and back sides after the start of initial calibration. While a color match is eventually achieved between the front and back sides, the user's demand may not be satisfied because, after calibration, the time during which the reading characteristics for the front and back sides do not match is too long.
The problems described above will be discussed in further detail with reference to FIG. 2A to FIG. 2C.
In FIG. 2A to FIG. 2C, the horizontal axis represents time, while the vertical axis represents the degree of color shift from a document level. A solid line indicates a reading characteristic of the first reading device 110 (hereinafter referred to as a front-side reading device), while an alternate long and short dashed line indicates a reading characteristic of the second reading device 111 (hereinafter referred to as a back-side reading device). Since the first reading device 110 is used in both single-side and double-side reading modes, the first reading device 110 may deteriorate more rapidly than the second reading device 111. However, this may not be applicable to a scanner with a different configuration or to a scanner in which the double-side reading mode is more frequently used.
FIG. 2A illustrates the case where calibration is performed with reference to the back-side reading device. The reading characteristic of the front-side reading device is greatly improved and can match that of the back-side reading device. However, there is a significant difference in color between a single-sided copy (1) made immediately before the calibration and a single-sided copy (2) made after the calibration.
FIG. 2B illustrates the case where calibration is performed with reference to the front-side reading device. The reading characteristic of the back-side reading device can match that of the front-side reading device. A difference in color between a single-sided copy (1) made immediately before the calibration and a single-sided copy (2) made after the calibration is reduced. However, as described above, since the front-side reading device is used in both the single-side and double-side reading modes, the front-side reading device is used more frequently than the back-side reading device. Therefore, durability of the front-side reading device deteriorates more rapidly than that of the back-side reading device. Making a correction with reference to the front-side reading device means that a reading device with a higher degree of color processing accuracy is adjusted to another reading device with a lower degree of color processing accuracy. In other words, a color shift from the document level is made larger (worse).
In the case of FIG. 2A, when a user who normally uses a single-side copying function makes a single-sided copy of a document that is the same as the one used several hours or several days ago, the resulting copy has a color different from that of the copy previously made. It is thus inevitable that the user will not be satisfied. In the case of FIG. 2B, a color match is achieved between the front-side reading device and the back-side reading device. However, their reading characteristics similarly deteriorate as described above. This deviates significantly from the original purpose of the calibration.
FIG. 2C illustrates a problem that arises when the technique of Japanese Patent Laid-Open No. 2005-244519 is applied to calibration between the front-side reading device and the back-side reading device. As illustrated, a difference in reading characteristic between the front-side reading device and the back-side reading device is only slightly reduced during calibration. It takes some time before the reading characteristic of the front-side reading device eventually matches that of the back-side reading device. In other words, even after calibration, a color shift remains large for a while, as indicated by a shaded area in FIG. 2C.